1. Field of the Invention
This invention relates to the field of current sensing circuits.
2. Description of the Related Art
Current is often measured by sensing the voltage across a sensing element which carries the current of interest. To minimize losses, it is desirable to have the sensing element""s resistance be as low as possible. However, this results in a small sensed voltage, which must be amplified before being used by other signal conditioning circuits such as analog-to-digital converters (ADCs).
Because the sensed voltage is so low, the amplifier""s characteristics, such as its input offset voltage, have a substantial effect on the accuracy of the current measurement. Chopper amplifiers are often employed, due to their low offset voltage. However, the small sensed voltage requires the amplifier to provide a large gain, which can result in noise in the sensed voltage having a significant impact on measurement accuracy. Moreover, the amplifier""s offset must be low when compared with the sensed voltage, to avoid degrading measurement accuracy.
A current measurement circuit is presented which overcomes the problems noted above, providing high accuracy current measurements for very small sensed voltages.
The measurement circuit is connected to a sensing element which carries the current of interest. The input terminals of a first pair of cross-coupled switches are connected to receive the sensed voltage, and the switches"" output terminals are connected to respective input capacitors. The input capacitors"" other terminals are connected to the input terminals of a second pair of cross-coupled switches, the output terminals of which are connected to the inputs of an amplifier having differential inputs and outputs. Respective feedback capacitors are connected between each of the amplifier""s outputs and inputs.
A control circuit operates the cross-coupled switches in accordance with a switching cycle, during which the connections between the sensing element and the input capacitors are interchanged, after which the connections between the input capacitors and the differential amplifier are interchanged. When so operated, the sensed voltage is sampled on the input capacitors and transferred to the feedback capacitors to produce a differential output voltage Vout from the differential amplifier. Several switching cycles are preferably performed, which causes the samples to be compounded on the feedback capacitors. This reduces the effects of noise on signal bandwidth, as well as reducing the differential amplifier""s gain requirement. When so arranged, the differential output voltage Vout is given by:
Vout=N*[(Cin/Cf)*2(I*R)]
where N is the number of consecutively performed switching cycles (i.e., switching cycles performed without discharging the feedback capacitors), Cin is the capacitance of the input capacitors, Cf is the capacitance of the feedback capacitors, R is the resistance of the sensing element, and I is the current of interest.
Additional features include the use of a pair of switches across the feedback capacitors to auto-zero the measurement circuit, the use of an attenuation network between the sensing element and the input capacitors to reduce the common-mode voltage applied to the measurement circuit, and various means by which error due to mismatch in the attenuation network can be reduced.